Question What computers have GF99?

[L13]

It took me a few reads, but I understand your approach. If we are defining decompression stress as the percentage of the allowable M value for the most loaded tissue compartment, then GF99 is a direct measure of decompression stress. OTOH, NDL and SurfGF are measures of decompression stress at the surface assuming an immediate standard rate direct ascent.

Ok, we agree on that?

But I think we disagree on the value of knowing your current GF versus a surface GF. What does it give you other than a rough minimum and maximum ascent indicator if for some reason your choose to ignore standard ascent rates?

Yes you could use it as a way to run a less conservative ascent if you decided to ignore your own gradient factors. But in that scenario, it's simply duplicating what SurfGF gives you. And the rest of the time, it seems to me that you would be better served with one of the other options; NDL in a planned no deco dive, or ceiling or +5 once in deco.

This seems to be Shearwater's take on it as well:
Evolution of Dive Planning - Shearwater Research (starting at the section titled "Real Time Management of Risk").

I think you are still confused. NDL is never a measure of decomession stress, it is not even a measure of N2 loading, it is a measure of time remaining at current(or planed in planing mode) depth until nitrogen loading will result in your GF99 exceeding your GF Lo/GF Hi line before you reach the surface, therefore requiring a deco stop.

SurfGF not a measure of decompression stress, it is a measure of what decompression stress (GF99) would be if you teleported to the surface. GF99 is similar to SurfGF in units of measure, but it is the current stress, right now, at the current depth.

Read the rest of the article you posted. It gives a good description of what GF99 would be during a decompression dive, and how it would be useful in general. Specifically, look at the graph labeled "Decompression Using Gradient Factors," the blue line is GF99. I would like to know where that blue line actually is during my dive.

Shearwater's take is actually my take, which is why I was asking if any other computers display GF99, or if I need to stick with Shearwater to get it.

 

[waterone]

The Garmin Descent Mk2 has a Surface Gradient Factor (SurfGF) data field. If you want to see it then you have to add it to one of the dive data screens, it's not displayed by default.

But I don't really see much point. It's not telling me anything useful.

Yes, it does and is why I said maybe Garmin will give an update for gf99 as I do not know of any others that even have surf gf. It was many updates and well over a year since release to get the surface gf. While you might not find it useful, I believe it is and have it added to the screen with 7 areas.

 

[inquis]

I think you are still confused. NDL is never a measure of decomession stress, it is not even a measure of N2 loading, [...] SurfGF not a measure of decompression stress

I don't believe he is. Both NDL and SurfGF are correlated with inert gas loading and therefore tissue stress to the layperson. This is, indeed different from the deco researcher concept of tissue stress as "degree of supersaturation", but mainly in a delayed sense. Dive to 100 ft and let NDL tick down to 0. Tissue stress (i.e., supersaturation) is zero at that instant, but 0 mins NDL obviously tells you it won't stay that way. (It's also impossible to get NDL=0 without tissue loading, so it seems like a decent proxy for "loading".) Similarly, SurfGF isn't the current stress, but it is -- by definition -- numerically equal to the tissue stress on the Buhlmann scale at the surface.

No argument that these are different "things" and care should be taken not to use them outside the realm of applicability. For instance, one shouldn't equate tissue loading at different depths when both are at 0 mins NDL.

 

[inquis]

I would like to know where that blue line actually is during my dive.

I'm curious as to why? What will you do (or not do) based on it? As I said, I don't find it useful during a dive as other metrics provide more direct feedback (for example, stop depth or ceiling on a deco dive), but I might be missing something.

 

[L13]

I'm curious as to why? What will you do (or not do) based on it? As I said, I don't find it useful during a dive as other metrics provide more direct feedback (for example, stop depth or ceiling on a deco dive), but I might be missing something.

Imagine a recreational dive to 110 feet. I stayed till NDL = 1min. Now I'm surfacing.

I could go directly to my safety stop, and my GF99 is a little less than my GF Hi(usually 75-85% depending on who I am diving with) and SurfGF. I pad my safety stop, but my peak deco stress is ~GF Hi, acceptable but:

I could come shallow, but not all the way to the SS, where my GF99 is 40-50%, tool around till it starts to drop off, slowly ascending to the SS with GF99 never exceeding 50%, and surface with a peak stress of <50% rather than 70%+. If I've go air, why not, it's safer, and mostly I just like it.

For Deco Dives, I can imagine many more scenarios, some involving urgencies or emergencies, where it would be useful. Obviously Shearwater agrees with me, and that is why they provide it .

 

[inquis]

Ok, so a self-implemented ceiling or an ad-hoc deco schedule. How do you know you have the air to complete it? I can easily see trying to stay under GF50, but then not actually having enough air to do that all the way to the surface. Obviously, it's still an improvement over what the computer is set to, so that's a plus.

 

[Manatee Diver]

I don't really look at GF99 as being very useful except in a contingency scenario, when I want to alter my ascent due to an emergency. For example going from my normal 60/80 to say 90/90 to get out of the water faster for whatever reason.

Now in both a recreational and tech context I do monitor Surface GF, mostly to extend my 20ft stop until my loading is at a lower level. I've not been bent, and I am trying to keep it that way. Another couple of minutes at 20ft with or without oxygen isn't likely to kill me.

 

[lowwall]

I think you are still confused. NDL is never a measure of decomession stress, it is not even a measure of N2 loading, it is a measure of time remaining at current(or planed in planing mode) depth until nitrogen loading will result in your GF99 exceeding your GF Lo/GF Hi line before you reach the surface, therefore requiring a deco stop.

SurfGF not a measure of decompression stress, it is a measure of what decompression stress (GF99) would be if you teleported to the surface. GF99 is similar to SurfGF in units of measure, but it is the current stress, right now, at the current depth.

I understand it just fine. SurfGF and NDL tell you your greatest tissue loading if you were to do an immediate ascent. Surf GF tells you directly, NDL tells you indirectly. They are actually equal at one point. When NDL ticks from 1 to 0, SurfGF will be ticking to your set GF High.

The Surfacing GF displays the GF that you would get if you were to ascend directly to the surface right now, without doing any stops.

If the SurfGF display shows 50, this means that if you were to ascend to the surface directly, your maximum tissue saturation would be 50% of the M-Value. I.e. well within your M-Value limit with almost no chance of DCS. If your SurfGF shows 150%, this means that a direct ascent to the surface would put you at 150% of your limit, and well over the M-Value limit with a very high chance of DCS. Finally, if your SurfGF shows 99, then a direct ascent to the surface would put you right on your M-Value limit and is equivalent to the NDL limit of a straight Buhlmann model.
[sections in italics are from the Shearwater article referenced above]

I still question the value of GF99.

Let's start with an NDL dive. It's your tissue loading at the surface that matters for avoiding DCS and lessening the effects of decompression stress. Either NDL time or SurfGF tell you this. The latter directly, the former by extrapolating from the time remaining and changes in time remaining as you change depths. GF99 doesn't tell you anything useful about your loading at surface. It's actually going to be misleading if you are using it for that since it's based only on the current most loaded tissue which will be one of the faster tissues (on a NDL dive). These tissues will rarely be the controlling tissue at the surface and are less important in DCS/deco stress anyway except perhaps in a runaway ascent.

Speaking of ascents, you could use it as a rough doublecheck on your ascent rate for either deco or NDL dives. But if that's a problem, your attention would be better spent on watching the ascent rate indicator rather than trying to extrapolate it from GF99.

On a deco dive, if you are diving your plan it doesn't matter at all. The only time it would be useful is if you want to abandon your plan and ascend faster than your selected gradient factors. Even then using SurfGF is a better way to do that for any dives where your gradient factors are requiring the stops, but you could actually make a straight ascent and stay below a GF of 99.

The same approach could be adopted with ascending all the way to the surface. In a critical emergency, the diver could edge up towards the surface watching their GF99 display and making sure that they stay close to, but not exceeding, their M-Value. However, this case can be managed more effectively using the Surfacing GF display feature.

That just leaves managing ad hoc emergency ascents in true deco dives. OK, I can see the value in that. Not really applicable to 99.??% of dives. But if that's a possibility in your diving then I can see why you'd make it a requirement for the purchase of your diving computer.

 

[L13]

Ok, so a self-implemented ceiling or an ad-hoc deco schedule. How do you know you have the air to complete it? I can easily see trying to stay under GF50, but then not actually having enough air to do that all the way to the surface. Obviously, it's still an improvement over what the computer is set to, so that's a plus.

The same way I know I have enough air to do a safety stop on any dive. If you have more than enough air to ascend to your safety stop, do the safety stop, and ascend to the surface, you can use that extra air to mitigate your peak decompression stress. If I'm headed to my safety stop and I have more than 1000 PSI, I definitely have gas to spare for this.

 

[L13]

. . .
Let's start with an NDL dive. It's your tissue loading at the surface that matters for avoiding DCS and lessening the effects of decompression stress. Either NDL time or SurfGF tell you this.

Right there you are wrong! Tissue loading is not the problem(in and of itself) and the problem is not limited to the surface, it is tissue supersaturation due to ambient pressure less than tissue dissolved gas pressure also known as tissue tensions and represented by GF99 that causes DCS. We are concerned about tissue loading because it will cause high tissue tensions when you reduce ambient pressure and therefore indirectly cause DCS. You can get DCS at any depth by ascending too rapidly to that depth with tissue loading from a deeper depth. If your GF99 is above 100, you are at risk of DCS no matter what depth you are at. If your SurfGF is above 100, you are only at risk if you ascend to quickly. DCS is not a surface only phenomenon. This is why on decompression dives you must do the deeper stops. The deeper stops prevent DCS at the shallower stops. You stay at each stop just long enough to decompress enough to prevent DCS at the next stop.